1. Field of the Invention
The present invention relates to a carburization heat treatment method of a vehicle workpiece that has a shape that is sensitive to heat deformation, such as an annulus gear of a vehicle transmission. The invention also relates to a vehicle workpiece carburized using the method.
2. Background Art
Generally, a vehicle transmission gear is a workpiece that is used for directly transferring engine power to a vehicle power system, and requires high fatigue strength. Thus, the transmission gear is carburized, quenched and thermally treated in order to improve fatigue strength through surface hardening.
Presently, a gas carburization heat treatment method results in the formation of an abnormal surface layer 15-25 μm thick which decreases durability on the surface of a workpiece and also results in abnormal heat deformation due to non-uniform cooling as quenching is carried out using an oil or a salt bath.
An annulus gear for an automatic transmission has a ring-type structure having internal teeth 1 as shown in FIG. 1. This structure is considerably sensitive or weak to heat in terms of the shape deformation. For example, when the annulus gear is subjected to gas carburization heat treatment, the ring shape becomes distorted or the shape of the teeth can be changed. As a result, the heat deformation causes abnormal assembly of an annulus gear into a transmission or abnormal noise in an assembled state.
Accordingly, the carburization heat treatment of the annulus gear is presently performed through a series of procedures of gas carburization to a surface, slow cooling, high-frequency heating, followed by plug quenching. The plug quenching is a process for quenching a workpiece which is held at various positions using a jig to prevent heat deformation. However, the plug quenching process is not applied to simultaneous treatment of a plurality of workpieces but instead to individual treatment of such workpieces, and thus can lead to lowered productivity and increased heat treatment costs.
Recently, the industry is increasingly using a vacuum carburization method. The vacuum carburization method, as compared to a gas carburization method using plug quenching, is advantageous in terms of high productivity and is considerably favorable for maintaining the shape of the teeth of, for example an annulus gear. However, when using the conventional vacuum carburization method, the shape of the annulus gear is considerably distorted upon cooling.
The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.